1. Field of the Invention
The present invention relates to a method of manufacturing an electronic device and, more particularly, to a method of manufacturing an electronic device having microlenses.
2. Description of the Related Art
An electronic device such as a solid-state image sensor or a liquid crystal display device can include microlenses for light collection. In a microlens forming method disclosed in Japanese Patent Laid-Open No. 10-148704, a mask layer having the three-dimensional shape of a microlens array is formed on a material layer, and the mask layer and the material layer are etched to form the microlens array in the material layer.
When forming microlenses by the method described in Japanese Patent Laid-Open No. 10-148704, the shape of the microlenses is determined by the three-dimensional shape of the mask layer. For this reason, when the three-dimensional shape varies, the microlens shape varies. The three-dimensional shape of the mask layer can vary in the exposure step of forming the mask layer due to halation of light that enters a resist layer to be used to form the mask layer and is reflected by a layer (for example, a color filter layer, a wiring layer, or an underlying substrate) under the resist layer. The variation in the microlens shape undesirably influences the electro-optical characteristics (for example, color unevenness, sensitivity, and f-number proportion) of a solid-state image sensor. Especially in a 3-chip solid-state image sensor such as 3-CCD or 3-CMOS type solid-state image sensor, the three-dimensional shape of the mask layer readily varies when forming it by photolithography. This is because the 3-chip solid-state image sensor includes no color filter, and the light that has passed through the mask layer in the exposure step is not absorbed by the color filter. Hence, a technique of stabilizing the microlens shape is particularly important for the 3-chip solid-state image sensor. The 3-chip solid-state image sensor is a device which includes three chips each having a plurality of photoelectric conversion units to sense light separated by an optical element, such as a prism, and which obtains one image based on the lights sensed by the three chips.
In addition, as the pixel size of the solid-state image sensor becomes smaller, the variation in the microlens shape more largely affects the optical characteristics. This is because the variation in the light collection efficiency caused by the variation in the microlens shape readily affects the optical characteristics when the area of the light receiving unit is reduced along with the pixel size reduction of the solid-state image sensor. Hence, the technique of stabilizing the microlens shape is also important for pixel size reduction of the solid-state image sensor. In a liquid crystal display device as well, as the pixel size becomes smaller, the variation in the microlens shape largely affects the optical characteristics.